What is "thin-film interference"

n. 1 (context uncountable optics English) interference of light caused by its reflection from both boundary of a thin film of a transparent material. 2 (context countable optics English) An instance of this interference.

Thin-film interference is a natural phenomenon in which light waves reflected by the upper and lower boundaries of a thin film interfere with one another to form a new wave. When the thickness of the film is a quarter-multiple of the wavelength of the light in the medium, the reflected waves from both surfaces interfere to "destroy" each other. Since the wave cannot be reflected, it is completely transmitted instead. When the thickness is a half-multiple of the wavelength within the medium, both reflections interfere to build each other up, increasing the reflected wave and reducing the transmission. This causes certain wavelengths (colors) to be intensified while others are attenuated.

Studying the light reflected or transmitted by a thin film can reveal information about the thickness of the film or the effective refractive index of the film medium. Thin films have many commercial applications including anti-reflection coatings, mirrors, and optical filters.